System and method for mapping directly accessible magnetic DASD storage to fixed block optical storage

ABSTRACT

A computer installation is programmed for storing data in a magnetic disk DASD (direct access storage device) in the form of a track number identified by DASD cylinder number (CC) and DASD head number (HH). A re-writable, multi-disk optical storage which stores information in a fixed-block architecture including a sequence of sectors having the form of spirals on optical disk surfaces is enabled to emulate magnetic DASD by calculation-based conversion of storage references to magnetic DASD tracks to fixed-block optical sectors.

This application is a continuation of application Ser. No. 08/275,934, filed Jul. 15, 1994, now U.S. Pat. No. 5,465,382, which is a continuation of application Ser. No. 07/870,024, filed Apr. 17, 1992, now abandoned.

BACKGROUND

This invention is related to the storage of data in a computer system, and more particularly to the emulation of direct access magnetic disk storage by means of fixed block architecture optical storage. More particularly, the invention concerns the conversion of a direct access magnetic storage data recording format to a direct access optical storage recording format to permit fixed block optical storage to emulate magnetic storage.

As is known, large computer installations employ direct access storage device (DASD) subsystems for fast, efficient, and economical storage of data in a device which records and retrieves data in response only to a reference of the data's location. The principal device which supports direct access storage is a disk. The most widely used disk technology in present-day computer installations employs a rigid magnetic disk on which data is written magnetically in a location format based upon parallel tracks in the form of concentric rings on each surface of the disk. Typically, a hard disk drive will contain one or more disks and will provide for the independent, direct accessing of each surface of each disk. In a multi-disk DASD, each surface of each disk is accessed by a respective head/actuator assembly so that identification of particular head identifies a particular disk surface. Within a multi-disk DASD, the set of all tracks of the same nominal circumference form a "cylinder". Thus, each track of a multi-disk DASD can be uniquely identified by a cylinder number (CC) and a head number (HH). A two-dimensional track address CC HH permits a track to be directly accessed by moving an identified head (HH) to the circumferential location of an identified cylinder (CC).

Fixed block architecture (FBA) optical storage is receiving increasing attention as a complement to, or a substitute for, magnetic DASD. FBA optical storage possesses a greater storage capacity than magnetic DASD in bits per storage area (areal density) and provides access to stored data at a speed that is somewhat slower than that provided by magnetic DASD. As is known, the use of magneto-optical technology supports a re-writable FBA optical storage which includes all of the storage features enjoyed by magnetic DASD.

Computer installation considerations may make it desirable to utilize re-writable FBA optical storage in place of magnetic DASD. However, the I/O (input/output) configuration of virtually all modern computer installations is based upon the magnetic DASD track format. FBA optical storage typically has only a single sequence of optical sectors in the form of a spiral "track" on each surface of each optical disk, implying only a single dimension for optical storage location format. In particular, data on an optical disk surface is addressed by only giving the sector number of the data. Therefore, in order to fully accommodate data storage in fixed-block optical storage for a modern computer installation, there is a need for mapping magnetic DASD track identification to FBA optical sector identification.

Mapping of one address form to another is taught in the prior art in the form of look-up tables. For example, U.S. Pat. No. 3,082,406 shows conversion of an external address location form to an internal address form in a data storage system by use of a look-up table. U.S. Pat. No. 4,947,367 teaches storage of magnetic tape formatted data on a WORM (write once, read many times) disk. In this patent, a look-up table is generated for addressing each tape, and the table includes a present WORM disk volume number storing a portion of the tape data as well as a next WORM disk volume number storing another part of the tape data. As is known, look-up tables are expensive in terms of storage space. Thus, as the capacity of DASD storage increases, track address emulation based upon look-up table address conversion would increase overhead by multiplying the storage space required for look-up tables, and might slow storage access due to table retrieval. Thus, the most desirable solution of the problem stated above should avoid the use of look-up tables to convert magnetic DASD track addresses to FBA optical sector addresses.

SUMMARY OF THE INVENTION

The system and method of this invention, when connected to a computer installation, permit data referenced in magnetic DASD track format to be stored on a re-writable fixed block optical storage without alteration of the computer system's standard DASD access protocol. Preferably, the system and method of the invention operate in response to data storage references made by a computer installation through a structured I/O subsystem in which each channel of a plurality of channels connects one or more I/O devices to a central computer. In such a subsystem, a specific DASD track is accessed through a channel program comprising a plurality of channel command words (CCW's). A storage access channel program for locating, reading, or writing data in a magnetic DASD track includes command information identifying a DASD by control unit address (CUA) and a particular track by cylinder number (CC) and head number (HH).

The invention is based on the inventors' critical observation that, in an FBA optical storage device having a plurality of optical disks, all sectors of the optical disks can be assigned numbers in one sequence of numbers denoting the relative position of any sector to all other sectors. The number of an optical sector in this sequence is termed a relative optical sector (ROS) number. This sequence enables the use of equations to calculate optical sector addresses in response to CC and HH numbers and eliminates the need to use cumbersome look-up tables. These equations reside in a controller connected to a channelized I/O system. The controller converts a magnetic DASD CCHH track address into fixed block optical sector address for the beginning of a sequence of optical sectors forming an emulated magnetic DASD track.

In particular, the invention is practiced in an information processing system including a computing facility for processing data and channel means connected to the computing facility for directing the storage and retrieval of data in a plurality of direct access storage devices (DASD), data being stored in the form of a substantially circular track denoted by a cylinder number (CC) and a head number (HH) in a DASD denoted by a channel unit address (CUA). In this environment, the invention is a combination for storing such data in a fixed block optical storage format and the combination includes:

an optical storage apparatus, including a plurality of optical disks, for storing data on the optical disks in the form of a substantially spiralled sequence of addressable optical sectors on one or more of the optical disks;

a control facility connected to the optical storage apparatus for directing the reading or writing of data at an optical storage address in response to channel control information including track address data which identifies a circular track; and

an address transform unit in the control facility for producing the optical storage address in response to CC, HH, and CUA data in the track address data, the optical storage address identifying a respective subsequence of optical sectors in the spiralled sequence having an information storage capacity at least equal to the information storage capacity of the circular track.

The invention thus enables a track of data in magnetic DASD format to be stored on, and retrieved from, fixed-block optical storage formatted into a sequence of sectors.

A principal objective of the invention, therefore, is to employ re-writable fixed block architecture optical storage to emulate magnetic storage.

A particular objective is to emulate the storage and retrieval of directly accessible storage tracks in fixed block optical sectors.

A significant advantage of the invention is provided in the translation of two-dimensional DASD track addresses into single-dimensional optical sector addresses without the use of look-up tables.

These and other objectives will be better understood from the following detailed description, taken together with the below-described drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a computer installation which emulates magnetic DASD in an optical storage apparatus of the fixed block type according to the invention.

FIG. 2 is a flow diagram showing how the invention accesses a particular optical sector containing data in an emulated track referenced by the computer installation of FIG. 1.

FIG. 3 is a schematic block diagram showing in greater detail elements of a controller included in FIG. 1.

FIG. 4 is a partial schematic drawing illustrating the emulation of a magnetic DASD track by relative optical sectoring in disk-based optical storage of the fixed block type.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1, a system block diagram is presented which illustrates the preferred embodiment of the invention, while FIG. 2 presents a flow chart illustrating the general operation of the embodiment. The system of FIG. 1includes a computer system 10 programmed for storage and retrieval of data in an existing magnetic DASD format. The computer system 10 is connected for recording and retrieval of data in a magnetic DASD storage by a channelized I/O subsystem. The connection of the computer system 10 with astorage subsystem is symbolized by the I/O channel 11. In the invention, data is recorded in, and retrieved from, a storage subsystem which emulates a magnetic DASD. The storage subsystem includes a re-writable multi-disk optical storage apparatus 12 in the form of an optical library system which is connected to the channel 11 through a library control unit

The storage subsystem receives a channel program on the channel 11 which isprovided to a channel/library management component 16. The channel/library management component 16 executes the channel program by providing channel commands to a command translation unit 18, magnetic DASD track identification data (CC" and HH) to an address transform unit 20 which transforms the magnetic DASD track address to an optical sector address, and buffer control information to a buffer manager 22. While performing its function, the management component 16 exhibits a standard magnetic DASD interface to the channel 11. Relatedly, a typical channel such as thechannel 11 can comprise an channel interface which connects a mainframe processor with a magnetic DASD control unit. Such a channel interface is described, for example, in "IBM System/360 and System/370 I/O Interface Channel to Control Unit Original Equipment Manufacturer's Information", Document GA22-6974-09 (1984) and "IBM System/360 and System/370 Direct Control and External Interruption Features Original Equipment Manufacturer's Information", Document GA22-6845-3 (1971), both available from the IBM Corporation, Armonk, N.Y. The corresponding magnetic DASD control functions for channelized I/O architecture can be understood with reference to the volume entitled "IBM 3990 Storage Control Reference", Document GA32-0099-3 (1989), also available from the IBM Corporation. The IBM 3990 Storage Control Reference assumes magnetic DASD in the form of the well-known IBM 3380 and 3390 direct access storage devices.

Preferably, the optical library system employed in the emulation of magnetic DASD is a magneto-optical disk library system of the type described in the reference volume entitled "Optical Disk Library System Technical Reference Manual", Document 5959-3559 (1990), available from theHewlett-Packard Company.

The commands for accessing, reading, and writing data in a magnetic DASD can be understood with reference to the IBM 3990 Storage Control Reference. These commands are provided over the channel 11 as a sequence of channel command words which form a channel program. The channel programis received by the management component 16. The commands included in the channel program are provided to the command translation apparatus 18 whichtranslates them into the appropriate corresponding commands for the opticallibrary system. Typical optical library system commands are found in the Optical Disk Library System Technical Reference Manual. Command translation is well understood in the art; in this invention it involves the recognition of a command sequence in a channel program and the mappingof the command sequence into an appropriate command sequence for the optical library system.

The channel/library management component 16 also derives from the channel program appropriate control information which is passed to the buffer manager 22 for the buffering of one or more tracks of data. In this regard, the buffer manager 22 controls a buffer 24 which may be provided, for example, in electronic read/write memory (not shown) included in the library control unit 14. The electronic memory buffer space is capable of storing at least one track of data. The typical configuration of a track is a sequence of records (REC0, REC1, . . . RECk, . . . ). Such configuration is well-known in the art and can be understood with reference to, for example, the count key data (CKD) and extended count keydata (ECKD) architecture employed in magnetic DASD's of IBM 3380 and 3390 type. ECKD track architecture is a self-describing data format. In this regard, all records in ECKD-formatted track begin with a Count field of fixed length which describes the length of following Key and Data fields. A track containing ECKD records can be searched by adding the lengths of Count, Key, and Data Fields to find the start of the next record. The lastrecord in the track is indicated by a bit in the count field of the last record. This architecture is employed by the buffer manager 22 in searching the memory image of a track buffered at 24.

A word concerning terminology. In the ECKD architecture, the data stored ina DASD is referred to as a "volume". Sometimes, the DASD itself is referredto as the volume; but, in this description, the first definition is preferred. The volume is indexed by addresses of the CCHHR type, where CC and HH have the same meanings given previously. The R portion of an address identifies a record and has the CKD form just discussed.

The important aspect of the library control unit 14 which concerns the invention is found in the sector address transform unit 20. The address transform unit 20 employs a calculation to translate the magnetic DASD track address to an optical sector address. The optical sector address includes an actual optical volume surface (AOVS) number and an optical sector (OS) number denoting the beginning of a sequence of optical sectorshaving an information storage capacity equal to that of a magnetic DASD track. In the preferred embodiment, an optical volume (OV) corresponds to a predetermined number of optical disks having the storage capacity to store a volume of data stored in DASD format. Each surface of each disk isidentified by the OVS number. The optical address includes the OVS and OS numbers, which, together, identify the optical sector on a particular optical disk surface where storage of a magnetic DASD track begins.

Translated commands and optical disk address data are provided to a small computer system interface (SCSI) driver 26 which provides SCSI formatted commands to the optical library system 12.

As shown in FIG. 1, the optical library system library includes an optical disk library 31 containing a plurality of magneto-optical disks, a plurality of optical drive units 32, and a disk autochange mechanism 34. The optical disks in the library 31 can be moved by the disk autochange mechanism 34 between respective storage slots and respective ones of the optical drives 32. Both the optical drives 32 and the disk autochange mechanism operate in response to SCSI commands received on the SCSI interface 30. In particular, each optical drive responds to a SCSI commandincluding an operand and an optical sector number. The optical drive denoted in the operand will access the optical disk engaged with it at theoptical address denoted in the relevant SCSI command field. The disk autochange mechanism 34 will move a disk from a disk storage location to aspecified optical drive in response to a SCSI command. The disk autochange SCSI commands interpret the OVS number by identifying the specific surfaceof a specific optical disk and moving that disk to engage the specific surface into engagement with an optical drive.

An emulated magnetic DASD track is either recorded or read by an optical drive utilizing the buffer 24. Alteration of a track buffered at 24 is directed by the buffer manager 22 at a respective record location in the buffered track. Since the invention buffers a magnetic DASD track 24, use of the invention in a library control unit such as the unit 14 to write a record requires the following command sequence: first, compute the opticalstorage address denoting the starting location (OVS and OS) of the emulatedtrack in the corresponding optical volume and load the optical volume surface or surfaces containing the track data; next, read an image of the track into the buffer 24; then, write the record at the proper record location in the track image; and, write the updated track image from the buffer 24 back to optical storage beginning at the starting location. Reading a record requires computing the starting location of the emulated track, reading an image of the track containing the record to the buffer 24 and then accessing the record in the track image and providing it to the computer system 10.

FIG. 2 illustrates the operational sequence observed by the library controlunit 14 in accessing the optical library system 12 in response to a reference by the computer system 10 to magnetic DASD storage. Initially, acommand is obtained from a channel command word (CCW) at 50, and the command is checked for validity at 52. If invalid, status is posted at 53 and returned to the computer system 10. If the command is valid, the operand or operands necessary to effect the channel command is (are) generated at 55, while translation of the magnetic DASD track address to the optical address is performed at 57. Before the operand or operands aregenerated at 55, the optical volume surface (OVS) number is checked at 58 against currently-mounted optical disks. If the optical volume surface is not mounted, a command is issued at 59 to the autochange mechanism to mount the volume. At step 60, the operand or operands generated at 55 and the optical sector (OS) number generated at 58 are combined at step 60 to perform the required read or write operations on the identified optical volume surface, which is accessed by concurrently-engaged optical driver. Following the read or write operation, status information, data, or statusinformation and data are returned to the computer system 10 at step 62 and the next channel command is awaited at 50.

FIG. 3 illustrates in greater detail specific elements of the command translation and parameter transform units 18 and 20 which are necessary toimplement steps 57, 58, and 59 of the operational sequence of FIG. 2. The command translation unit 18 includes the operand field of a channel command register 64, an operand generation unit 66, and the operand field of a SCSI command register 70. The parameter transformation unit includes the parameter field of the channel command word register 64, an parameter calculation unit 67, a register 68 for holding a current optical volume surface number, a comparison unit 69, and the parameter field of the SCSI command register 70. A sequence control unit 63 generally controls the flow of data and synchronizes the operations of these elements.

Initially, a channel command is loaded into the register 64 and the operandfield is provided to an operand generation unit which maps a channel command operand to one or more optical library command operands. The output of the operand generation unit 66 feeds the operand field of the SCSI command register 70. The parameter field of the channel command register 64 contains the control unit address for accessing a particular magnetic DASD, and the CC and HH numbers which identify a particular trackin the addressed DASD. These numbers enable the parameter calculation unit 67 to calculate the optical sector and optical volume surface numbers necessary to map the addressed magnetic DASD track to optical storage. Thecurrent optical volume surface being accessed by the optical driver of interest is identified by the contents of the register 68 and compared against the calculated OVS number at 69. If the numbers are unequal, the compare unit 69 issues a Change OVS control signal which loads the calculated OVS number into the register 68 and prompts the operand generation unit 66 to generate one or more command operands for the autochange mechanism 34 which are necessary to load the calculated opticalvolume surface. The sequence control unit 63 ensures that the current channel command is retained in the register 64 until any necessary opticaldisk changes have been completed and ensures that the optical sector and optical volume numbers calculated by 67 are phased into the parameter fields of the register 70 in synchronism with the appropriate command operands. The SCSI commands are assembled and forwarded from the register 70 and include operands generated at 66 and OS or OVS numbers calculated at 67.

The parameter calculation unit 67 of FIG. 3 determines optical sector and optical volume surface numbers in response to values for CC, HH, and CUA according to the following equations:

    f(ROS)={(DCN×NEH)+DHN}×OST                     (1)

where:

∘ ROS=Relative Optical Sector Number of an emulated magnetic volume

∘ DCN=DASD Cylinder Number (CC)

∘ NEH=Number of Emulated Heads (constant)

∘ DHN=DASD Head Number (HH)

∘ OST=Number of Optical Sectors per Emulated Track

OST=ETL÷OSL (Rounded up to next whole number)

∘ ETL=Emulated Track Length in Bytes (constant)

∘ OSL=Optical Sector Length in Bytes (constant)

    f(AOS)=remainder(ROS÷OSV)                              (2)

where:

∘ AOS=Actual Optical Sector Number within the Actual Optical Volume Surface

∘ OSV=Number of Optical Sectors per Optical Volume Surface

∘ ROS=Relative Optical Sector Number of the emulated magnetic volume

    f(AOVS)=integer(ROS÷OSV)+(CUA×VPU)               (3)

where:

∘ AOVS=Actual Optical Volume Surface

∘ OSV=Number of Optical Sectors per Optical Volume Surface

∘ ROS=Relative Optical Sector Number of the emulated magnetic volume

∘ CUA=Control Unit Address (Host device sub-channel address)

∘ VPU=Number of Optical Volume Surfaces per emulated magnetic Volume

These equations assume that the volumes to be stored are of equal size in byte storage capacity, as are the tracks in the volumes. In equation (1), a relative optical sector number is calculated which establishes a sequence of a predetermined number of optical sectors and which indicates where, within that sequence, the beginning of any emulated magnetic DASD track occurs with respect to any other magnetic track. For example, consider the IBM 3390 magnetic DASD, Model 2. This device has heads numbered 0-14, 2226 cylinders, numbered 0-2225, and 56,664 bytes per track. Assuming an optical sector length of 1024 bytes (which is equal to the sector length for the optical disk library system referenced above), the number of optical sectors per track (OST) is 56. Consider now the relative optical sector numbers for the following emulated tracks: CC=0, HH=0; CC=0, HH=1; CC=0, HH=14; CC=1, HH=0; and CC=2225, HH=14. As Table I shows, all emulated tracks in the first cylinder, CC=0, are mapped into the first 840 relative optical sectors (0-839) and the first emulated track of the second cylinder (CC=1) begins immediately thereafter at relative optical sector 840. The last emulated track in the last cylinder (CC=2225, HH=14) begins at relative optical sector number 1,869,784 and ends, 56 optical sectors later at relative optical sector number 1,869,840. The implication is that a volume stored in a Model 3390 magnetic DASD is emulated in the referenced optical library system by an optical sequence of 1,869,840 consecutive sectors.

The actual optical address which locates any respective track in the emulated volume stored in the IBM 3390 magnetic DASD is given by equations(2) and (3). The address consists of the actual optical sector number (AOS)on an actual optical volume surface number (AOVS). Relatedly, the calculations discussed above with respect to relative optical sector number showed that 1,869,840 optical sectors are needed to emulate a magnetic volume stored in the 3390 device. Since the optical disk library system referenced above provides a storage capacity of 312,480 sectors peroptical disk surface, it can be appreciated that a total of six optical disk surfaces (three optical disks) is necessary to form an optical volumewhich will emulate the 3390 magnetic DASD-stored volume. Equation (3) associates a respective set of six consecutively-numbered optical volume surfaces with a respective magnetic DASD identified by a control unit address (CUA). In the channelized I/O architecture discussed above, control unit addressing supports the identification of up to 64 individualmagnetic DASD volumes, which are numbered 0-63. Given that OSV=312,480 and VPU=6, the actual optical sector and actual optical volume surface numbersgiven in Table I assume an emulated magnetic DASD volume whose volume CUA is 10. For this emulated magnetic volume, the first track of the first cylinder (CC=0, HH=0) begins at optical sector 0 on optical volume surface60. The last emulated track (CC=2225, HH=14) begins at optical sector 307,384 on optical volume surface 65, the last in the set of six optical volume surfaces numbered 60-65 which support the emulation of the magneticDASD at channel address 10.

                  TABLE I                                                          ______________________________________                                         EMULATED TRACK     ROSN     AOS      AOVS                                      ______________________________________                                         CUA = 10, CC = 0, HH = 0                                                                          0        0        60                                        CUA = 10, CC = 0, HH = 1                                                                          56       56       60                                        CUA = 10, CC = 0, HH = 14                                                                         784      784      60                                        CUA = 10, CC = 1, HH = 0                                                                          840      840      60                                        CUA = 10, cc = 2225, HH = 14                                                                      1,869,784                                                                               307,384  65                                        ______________________________________                                    

FIG. 4 illustrates how the invention supports emulation of magnetic DASD ofthe 3390 type with a re-writable fixed-block optical storage apparatus of the type described above. In FIG. 4, assume a control unit 79 having a control unit address CUA serves a 3390-type magnetic DASD indicated by reference numeral 80. In the magnetic DASD 80, a track positioned in a cylinder 81 (CC) and served by a magnetic head/actuator 82 (HH) is indicated by reference numeral 83 and identified by CCHH. Given CUA, CC, HH, the address calculation unit 67 of FIG. 3 derives the ROSN, and then the actual optical sector and optical volume values. These values are found on a respective surface 84 of an optical disk 86 which contains a spiral track 89 of optical sectors. It should be noted that the actual optical sector value (OS) calculated by the address calculation unit 67 isthe number of the optical sector where the emulated track 83 begins; of course, the ending optical sector is calculated simply by adding to the beginning optical sector value the number OST, which is the number of optical sectors per track. Thus, the SCSI command built according to FIG. 3 for reading the emulated track 83 can be given two values by the addresscalculation unit 67: the beginning and ending optical sectors for the emulated track. These sectors are indicated by the values OS_(b) for thebeginning optical sector and OS_(e) for the ending optical sector.

According to the description given above, it is possible now to emulate magnetic DASD in fixed-block optical storage by mapping magnetic DASD track addresses to optical sector addresses without modification to eitherthe channel architecture for the magnetic DASD or the fixed block architecture of the optical storage apparatus. The address translation provided by the invention in combination with re-writable optical storage underpins a highly useful emulation of erasable magnetic DASD.

Although the invention has been described in terms of a specific embodiment, the inventors contemplate that modifications and substitutionsto various components of the invention would occur to the person of ordinary skill in the art and, therefore, would be within the scope of theinvention, which is to be limited only by the claims which follow. 

We claim:
 1. An interface to exchange data between a computer and an optical library by receiving DASD-based commands from the computer, translating the DASD-based commands into sector-based commands, and providing the sector-based commands to the optical library:wherein each said DASD-based command includes a DASD-based operand including a storage operation compatible with a DASD storage device that stores data in multiple disks each having a multiple concentric tracks of varying sizes, said DASD-based command also including a DASD-based address of the DASD storage device; wherein each said sector-based command includes a sector-based operand including a storage operation compatible with the optical library that stores data in multiple disks each having at least one surface containing a single continuous spiral of multiple storage sectors, said sector-based command also including a sector-based address specifying a surface of an optical disk and a sector; and wherein the optical disks are selectively loaded by an autochange mechanism;said interface comprising: an address transform unit, including:a DASD-based address register to receive from the computer and store an input DASD-based address of a DASD-based command; a parameter calculation unit to mathematically calculate a sector-based target address uniquely corresponding to the input DASD-based address, said sector-based target address including a target surface and a target sector of the target surface; a current optical surface register to store a representation of an optical disk surface presently loaded by the autochange mechanism; a comparison unit to compare contents of the current optical surface register and the target surface; a sector-based address register to receive and store the sector-based target address from the parameter calculation unit, and provide the sector-based target address to the optical library; a command translation unit, including:a DASD-based operand register to receive from the computer and store an input DASD-based operand of a DASD-based command; an operand generation unit to generate a sector-based operand compatible with the optical library to operate the autochange mechanism, wherein said sector-based operand is selected to load the target surface when the comparison unit indicates the target surface is not currently loaded, and otherwise to translate the DASD-based operand from the DASD-based operand register into sector-based target operand compatible with the optical library; and a sector-based operand register to receive and store sector-based operands from the operand generation unit, and provide the sector-based operands to the optical library.
 2. The interface of claim 1, wherein the DASD-based address includes a cylinder number and a head number.
 3. The interface of claim 1, further comprising a SCSI interface electrically connected to the sector-based operand register and the sector-based address register.
 4. The interface of claim 1, further comprising an I/O channel electrically connected to the DASD-based operand register and the DASD-based address register.
 5. The interface of claim 1, wherein the autochange mechanism is configured to load a single optical disk at one time.
 6. The interface of claim 1, wherein the sector-based operand register and the sector-based address register are concatenated to form a single register.
 7. The interface of claim 1, wherein the DASD-based operand register and the DASD-based address register are concatenated to form a single register.
 8. The interface of claim 1, wherein each track has a storage capacity of multiple storage sectors.
 9. The interface of claim 1, further comprising a buffer, electrically connected to the computer and the optical library, to exchange data between the optical library an the computer in accordance with contents of the sector-based operand register and the sector-based address register.
 10. The interface of claim 9, further comprising a buffer manager electrically connected to the buffer.
 11. The interface of claim 1, further including a sequence control unit, electrically connected to the operand generation unit.
 12. The interface of claim 11, wherein the sequence control unit is also electrically connected to the DASD-based operand register, the comparison unit, and the sector-based operand register.
 13. The interface of claim 11, wherein the sequence control unit is also electrically connected to the buffer manager.
 14. The interface of claim 1, further including the DASD storage device.
 15. An address transform unit for use in interfacing a computer system and an optical library including an autochange mechanism to selectively load optical disks of the optical library, said computer system being programmed to send and receive DASD-based memory access commands compatible with a DASD storage device that stores data in multiple disks each having multiple concentric tracks of varying sizes, said optical library being programmed to send and receive sector-based memory access commands compatible with multiple disks of the optical library each optical disk having at least one surface containing a single continuous spiral of multiple storage sectors, said address transform unit comprising:a DASD-based address register to receive from a computer and store a DASD-based address of a DASD-based memory access command; a parameter calculation unit to calculate a sector-based target address of a storage location in the optical library uniquely corresponding to the DASD-based address under a pre-defined relationship; a first status register to store a representation of optical disk addresses on all optical disks presently loaded by the autochange mechanism; a comparison unit to determine whether the optical disk addresses stored in the first status register include the sector-based target address; and a sector-based address register to receive and store the sector-based target address from the parameter calculation unit, and provide the sector-based target address to the optical library.
 16. The address transform unit of claim 15, further comprising a command translation unit, including:a DASD-based operand register to receive from the computer and store a DASD-based operand of the DASD-based memory access command; an operand generation unit to generate a sector-based operand compatible with the optical library to operate the autochange mechanism, wherein said sector-based operand is selected to load an optical disk containing the sector-based target address if the comparison unit indicates the target address is not present on an optical disk currently loaded, and otherwise to translate the DASD-based operand from the DASD-based operand register into a sector-based target operand compatible with the optical library; and a sector-based operand register to receive and store sector-based target operands from the operand generation unit, and provide the target sector operands to the optical library.
 17. An interface to exchange data between a computer and an optical library by receiving DASD-based commands from the computer, translating the DASD-based commands into sector-based commands, and providing the sector-based commands to the optical library,wherein each said DASD-based command includes a DASD-based operand including a storage operation compatible with a DASD storage device that stores data in multiple disks each having a multiple concentric tracks of varying sizes, said DASD-based command also including a DASD-based address of the DASD storage device, wherein each said sector-based command includes a sector-based operand including a storage operation compatible with the optical library that stores data in multiple disks each having at least one surface containing a single continuous spiral of multiple storage sectors, said sector-based command also including a sector-based address specifying a surface of an optical disk and a sector; and wherein the optical disks are selectively loaded by an autochange means;said interface comprising: address transform means, including:a DASD-based address register to receive from the computer and store an input DASD-based address of a DASD-based command; parameter calculation means to mathematically calculate a sector-based target address uniquely corresponding to the input DASD-based address, said sector-based target address including a target surface and a target sector of the target surface; a current optical surface register means to store a representation of an optical disk surface presently loaded by the autochange mechanism; a comparison unit for comparing contents of the current optical surface register and the target surface; a sector-based address register means for receiving and storing the sector-based target address from the parameter calculation means, and provide the sector-based target address to the optical library; command translation means, including:DASD-based operand register means to receive from the computer and store an input DASD-based operand of a DASD-based command; operand generation means to generate a sector-based operand compatible with the optical library to operate the autochange means, wherein said sector-based operand is selected to load the target surface when the comparison means indicates the target surface is not currently loaded, and otherwise to translate the DASD-based operand from the DASD-based operand register means into a sector-based target operand compatible with the optical library; a sector-based operand register means to receive and store the sector-based target operands from the operand generation means, and provide the sector-based target operands to the optical library.
 18. The interface of claim 17, wherein the DASD-based address includes a cylinder number and a head number.
 19. The interface of claim 17, further comprising a SCSI interface electrically connected to the sector-based operand register means and the DASD-based address register means.
 20. The interface of claim 17, further comprising an I/O channel electrically connected to the DASD-based operand register means and the DASD-based address register means.
 21. The interface of claim 17, wherein the autochange means is configured to load a single optical disk at one time.
 22. The interface of claim 17, wherein the sector-based operand register means and the sector-based address register means are concatenated to form a single register means.
 23. The interface of claim 17, wherein the DASD-based operand register means and the DASD-based address register means are concatenated to form a single register means.
 24. The interface of claim 17, wherein each track has a storage capacity of multiple storage sectors.
 25. The interface of claim 17, further comprising a buffer, electrically connected to the computer and the optical library, to exchange data between the optical library an the computer in accordance with contents of the sector-based operand register means and the sector-based address register means.
 26. The interface of claim 25, further comprising a buffer manager electrically connected to the buffer.
 27. The interface of claim 17, further including a sequence control means, electrically connected to the operand generation means.
 28. The interface of claim 27, wherein the sequence control means is also electrically connected to the DASD-based operand register means, the comparison means, and the sector-based operand register means.
 29. The interface of claim 27, wherein the sequence control means is also electrically connected to the buffer manager.
 30. The interface of claim 17, further including the DASD storage device. 